**1. Introduction**

In recent years, nanofluids in various industries have been considered because of their ability to improve heat transfer. Nanoparticles are typically used in the form of nanofluids that consist of nanoparticles and a base fluid. The effective thermal conductivity is increased by the nanoparticles in a nanofluid, which can significantly improve heat transfer [1]. Today, nanoparticles are present in nearly all science and engineering branches. Nanofluids are colloidal dispersions of nanometer-sized materials (stable metals, metal oxides, oxide ceramics, metal carbides, etc.). In particular, Al2O3 nanofluids have been widely investigated and used due to their high productivity, higher thermal properties, and stability [2]. Accordingly, many studies have been done on Al2O3 nanofluids in heat transfer applications [3, 4].

The characteristics of thermophysical for Al2O3 are essential to be examined to find how Al2O3 nanoparticles influence the heat transfer and hydraulic properties of nanofluids [5]. Pak and Cho [6] measured the thermophysical characteristics of TiO2 and Al2O3 nanofluids. Viscosity was determined by changing the volume fraction from 1–10%. The results prove that the density and viscosity increase with the volume fraction. Also, viscosity is approximately independent of the shear rate. Agarwal et al.

[7] produced Al2O3 nanoparticles and analyzed thermal conductivity. It is dispersed in ethylene glycol or water with different concentrations. At a temperature of 1000 C, the synthesized Al2O3 nanoparticles are primarily stable and improve thermal conductivity. Kiruba et al. [8] investigated the effect of adding polyethylenimine on the rheological properties of Al2O3 nanofluids. It was proposed that adding polyethylenimine makes the viscosity independent of the fluid temperature and improves gel formation. Kumar et al. [9] investigated the effect of temperature changes (30 C, 40 C, 50 C) on the thermal conductivity of Al2O3 nanofluid. The results showed that increasing the volume fraction of nanoparticles in water from 0.01 to 0.08% vol.% improves thermal conductivity, and with increasing temperature, thermal conductivity is further improved.

In a nuclear reactor, heat is released through fission in the fuel rod. Heat is then transferred to the cladding through thermal conduction from the fuel surface [10]. From the cladding surface, heat is transferred to the coolant through convection. The heated coolant is passed to heat exchangers, where steam is generated to operate a power cycle. Nuclear fuel rods are employed as heat sources. Most nuclear reactors run on fuel rods containing the uranium-238 and uranium-235 isotopes. Fuel fission rate and thermal neutron flux affect heat generation in a nuclear reactor. Also, by changing the mass flow rate of the reactor coolant and the temperature difference across the core, the thermal power produced by a reactor changes directly [11]. Uranium is usually in the form of pure metal, in a compound such as U02, uranium oxide, or in the form of an alloy with another metal such as aluminum or zirconium (in the form of rectangular plates or long cylindrical rods).

Optimal properties of a fuel that must be fissile include high thermal conductivity, good corrosion resistance, good mechanical resistance at high temperatures, good corrosion resistance, and a high limiting temperature for operation [12].

The fuel rod surface heat flux is the most critical factor in developing or designing a new fuel rod because it must be measured as the maximum fuel centerline temperature. As shown in **Figure 1**, the generated heat in the fuel pellet flows into a coolant through only one rod surface in the solid-type fuel, whereas the annular fuel has two surfaces and a dual coolant channel. Thus, the heat generated in the annular fuel can flow into the inner or outer surface.

The internally and externally cooled annular fuel increases power density in the standard Westinghouse PWR while keeping or increasing the safety margin [13, 14]. In 2017, the thermal–hydraulic analyses of externally and internally cooled annular fuel were investigated. This study identified the geometry that allows the largest possible power density while maintaining or increasing the minimum departure from

**Figure 1.** *Schematic of solid (left) and annular fuel (right).*

### *The Effect of Al2O3 Concentration in Annular Fuels for a Typical VVER-1000 Core DOI: http://dx.doi.org/10.5772/intechopen.105192*

nucleate boiling ratio (MDNBR) margin in current PWRs [15]. The DUO THERM program investigated the design analysis of a dual-cooled annular fuel. The program studied pellet and cladding deformations caused by irradiation and power variation to predict the inner and outer heat fluxes and fuel temperature. Using the DUO THERM program, the fuel temperature and heat flux were estimated for a reference annular fuel design. The results showed that the annular fuel heat flux was significantly influenced by the inner and outer gap conductance behaviors. At zero burnup, the heat flux of the inner cladding was maximum. Furthermore, the location of maximum fuel temperature was altered by gap conductance asymmetry [16].

KAERI (Korea Atomic Energy Research Institute) developed a power-uprated annular fuel reloading. This project aimed to develop annular fuel that can be used in the current OPR-1000 pressurized water reactor. It was considered a 12 12 annular fuel to evaluate the thermal–hydraulic performance and compared its efficiency against a 16 16 cylindrical fuel assembly. The aim of this study was to evaluate dualcooled annular fuel at normal power for OPR-1000 and finally to measure the possibility of 120% core power. The results illustrated that annular fuel has sufficient margin available on DNB and fuel pellet temperature relative to cylindrical fuel [17, 18]. Two significant safety coefficients (prompt reactivity and power coefficients) of a typical PWR were considered for the annular fuel core using the MCNP-5 code. The optimized 13 13 arrays performed the calculations for annular configuration [19]. Moreover, an internally and externally cooled annular fuel was simulated and adapted to a typical VVER-1000 reactor. The results demonstrated that an annular pin configuration, called annular-8, was suggested based on the fully neutronics and MDNBR evaluations [20].

One of the main components of a nuclear reactor is the cooling system. The coolant enters the core at a low temperature and exits at a high temperature after the fission energy is transferred to it. Heat is then transferred from the high-temperature fluid to other thermodynamic components, and eventually, electrical power is generated. High-performance cooling plays a vital role in the efficiency and security of the nuclear power plant. Consequently, the investigation of the effects of nanofluids has been one of the important subjects in recent years. Nanofluids have been found to possess improved thermo-physical properties such as thermal conductivity. Several studies have shown that nanofluids have great potential for increasing heat transfer rates in various application cases while incurring either little or no penalty in pressure drop [21–23].

In an experimental study, it was investigated different volume concentrations of Al2O3 nanofluid flowing in a horizontal shell and tube heat exchanger. The results show that the nanofluid's convective heat transfer coefficient is slightly higher than the base fluid at the same mass flow rate [24]. Also, the nanofluids were simulated as the coolant in the VVER-1000 reactor core to analyze thermal–hydraulic performance using the porous media approach. The results displayed that the temperature of the coolant increases with the concentration of nanoparticles. Due to nanofluids' higher heat transfer coefficient than pure water, the coolant flow rate can be reduced [25].

In the present study, the effect of Al2O3 with various volume percentages in annular fuels on a typical VVER-1000 core was investigated. The FLUENT 6.3.26 code is used together with Gambit mesh generation software to model the annular fuels. The prediction–correction method with a SIMPLEC algorithm is applied to numerical solutions. The k-ε model is used to consider the turbulence effect. The cosine form of generated heat in the axial direction of the fuel rod is defined using the User Defined Function (UDF). Firstly, the simulation accuracy is proved by comparing with other

studies and the final safety analysis report (FSAR) for the solid fuel with the concentration of nanoparticles. Then, it is simulated annular fuels with various volume percentages of nanofluid. Finally, the temperature distribution in the fuel, clad, and coolant with nanoparticles and pure water concentration is presented.
